The invention relates to scintillators, and more particularly to fast, bright inorganic scintillators based on direct-gap semiconductors.
Inorganic scintillators have been used as radiation detectors for x-rays, gamma rays, and neutrons, for over a century. The history of inorganic scintillators is described in S. E. Derenzo et al., “The Quest for the Ideal Inorganic Scintillator,” Nuclear Instruments and Methods in Physics Research A 505 (2003) 111–117, which is herein incorporated by reference.
Table I lists the best available prior art scintillators and compares them to the ideal.
TABLE IPropertiesBGONaI:TlBaF2LSOLaBr3:CeIdealphotoelectric fraction*0.430.180.190.340.14>0.43Density7.13.74.97.45.3>7Photons/MeV8,20040,0001,80020,00061,000>100,000Energy resolution**13%6%10%11%3%<3%Decay time (ns)300230<14035<1Photons/ns/MeV***2618020005001750>100,000*σphoto/(σphoto + σCompton) at 511 keV**fwhm at 511 keV***Photons/ns/MeV = (photons/MeV)/(decay time)
FIG. 1 shows the physical processes active in the various common prior art scintillators. These include self-trapped excitonic scintillators such as BaF2; scintillators with activator ions such as NaI(Tl), CsI(Tl), LSO (Lu2SiO5:Ce), and LaBr3:Ce: self-activated scintillators such as BGO (Bi4Ge3O12); and core-valence scintillators such as BaF2. Certain phosphors have ultra-fast decay times, e.g. 0.7 ns for ZnO:Ga and 0.2 ns for CdS:In. Unfortunately, no known inorganic scintillator is both bright and fast.
Direct-gap semiconductors are fast, luminous scintillators when cooled to cryogenic temperatures, but have greatly reduced luminosity at room temperature due to carrier trapping on non-radiative centers (crystal defects and impurities). The need for cryogenic temperatures limits their use. A scintillator that is bright and fast at room temperature is desired.
Positron Emission Tomography (PET) is able to measure the concentration of labeled compounds in the human body as a function of time and is an efficient and accurate method for measuring regional biochemical and physiological functions. It has been useful for the study of heart disease, brain disease, and cancer. However, PET has been limited by the timing and energy resolution of available scintillators. A scintillator with properties close to the fundamental limits of 1 ns decay time, 200 ps fwhm timing resolution, and 3% fwhm energy resolution would substantially improve PET performance. Thus a new inorganic scintillator for detecting 511 keV photons with exceptional detection efficiency (like BGO), response time (1 ns), and energy resolution (3% fwhm) would allow septaless time-of-flight PET to be achieved. Fast bright scintillators could also enable an efficient time-of-flight gamma detector useful in other fields, such as nuclear physics, high energy physics, and astrophysics.